Your question is not clear? Title : "When to use “y/en” or “lui/leur”?" (which in itself represents two different questions). Then your final question is "What's the difference between these "à"?". What exactly are you asking about?
– LaureJun 18 '16 at 6:25

4 Answers
4

À is a preposition used to indicate destination. It is very often used after a verb indicating movement to introduce the destination.
There is no difference in the use of à in your two sentences.

Je téléphone à mes amis.

À introduces the person you are talking to over the phone. (Beware that in French téléphoner introduces an indirect object*, so we have téléphoner à just like we have parler à.

Il va à Paris.

À introduces the place you are going to.

In both cases when you replace the complement "à mes amis" / "à Paris" by a pronoun (leur used for an indirect object, y for a place) the pronoun goes before the verb and à disappears, indirect objects pronouns leur and y contain the idea of destination conveyed by à.

First, let's distinguish two types of complements that can be marked with à: the first are locative adjuncts, for example "à Lille" in "j'ai rencontré les parents de Sabri à Lille". They're not an obligatory complement, they could be replaced by any locative phrase headed by another preposition (dans le centre commercial, en forêt, chez eux, sur la plage, etc.), and they can qualify any verb. Those locative adjuncts are always pronominalised by y or by an adverb like là.

The second are indirect objects that are an intrinsic part of a verb's valence. For example donner (give) has two intrinsic complements: donner [quelque chose] [à quelqu'un] and those complements deeply affect the meaning of the verb ("sauter sur quelqu'un" et "sauter quelqu'un" mean something extremely different from each other). Those behave in a much more interesting manner with regard to the pronominalisation of their à-complements.

I'd categorize them in tree groups. One of them isn't sensitive to the animacy of their complement, the other two are:

This is rarely done in speech however, we'd rather use là (Je leur ai téléphoné là).

Verbs that (underlyingly) always take y

Those include s'interesser à (take an interest in), faire attention à (pay attention, be careful about) or penser à (thinking about). When the à-object is inanimate, their pronoun is y. When it's animate, we use "à + a strong pronoun"

For both of those categories, the strong pronoun à ça can be used instead of y.

How to know whether a verb belongs to which class? Unless you know Latin (lui/leur verbs tend to descend from verbs that took dative complements, y verbs from those that took prepositional phrases with ac/ab/ad), you'll have to memorize it as part of your knowledge of the verb.

You use leur when you are referring to them, where them is some people or some animals. You use y when you are referring to it an inanimate object. For example I respond to them where the them is some people is je leur réponds but if the them is some questions then the French is je y réponds. There's more information on this grammar point at https://rapidefrench.com/grammar/13/

@Kii It was a specific answer to his question, where the second example was concerning a place. The pronoun Y can be used when you have à + antécédent : tu peux te fier à lui : tu peux t'y fier , Tu gagnerais à le connaître ,Tu y gagnerais_, but the answer was to help him ditinguish the two examples !
– KoblenzJun 18 '16 at 11:11

2

Your first sentence is just simply wrong by virtue of incompleteness. J'en viens deals with a place, while j'y pense does not.
– temporary_user_nameJun 20 '16 at 3:18

@Aerovistae If you read the examples, his problem is not to differentiate between when he have to use y instead of en but to differentiate when he have to use y/en (on of them) or lui/leur. Je téléphone à mes amis Does he have to say Je l'y/en téléphone, or to say _Je leur/lui téléphone.
– KoblenzJun 20 '16 at 12:08